Bidirectional long cavity semiconductor laser for improved power and efficiency

ABSTRACT

The invention relates to bi-directional long-cavity semiconductor lasers for high power applications having two AR coated facets (2AR) to provide an un-folded cavity with enhanced output power. The lasers exhibit more uniform photon and carrier density distributions along the cavity than conventional uni-directional high-power lasers, enabling longer lasers with greater output power and lasing efficiency due to reduced longitudinal hole burning. Optical sources are further provided wherein radiation from both facets of several 2AR lasers that are disposed at vertically offset levels is combined into a single composite beam.

TECHNICAL FIELD

The present invention generally relates to semiconductor lasers, andmore particularly relates to bidirectional long cavity semiconductorlasers that emit laser light from two low-reflectivity facets forproviding higher overall output optical power and improved efficiency.

BACKGROUND OF THE INVENTION

Semiconductor lasers with high output optical power are of interest tomany application, including but not limited to optical pumping of fiberand solid state lasers, frequency doubling, and material processing.FIG. 1 schematically shows, in top view, a typical high-powersemiconductor laser, which has a high-reflectivity (HR) coated backfacet (BF) 1 and anti-reflection (AR) coated front face (FF) 3 in orderto concentrate all output optical power of the laser into a singleoutput beam from the front facet 3.

Historically, increasing the cavity lengths L and, where multimodegeneration is acceptable, the width of the active region, was found toimprove the laser output optical power due to better heat dissipationand reduced injection current density. However, the maximum output ofconventional high power semiconductor lasers has been limited torollover power levels of approximately 20 W for broad-area 100 um widthlasers, and less for single-mode narrow-area lasers with the waveguidewidth of a few microns. Accordingly, for applications requiring evengreater optical power, optical sources have been developed whereinradiation from multiple high-power lasers is combined in one or twooutput beams, see for example U.S. Pat. No. 8,427,749 and U.S. Pat. No.8,437,086, both of which are assigned to the assignee of the presentapplication and are incorporated herein by reference.

However, there is still a need to provide semiconductor lasers andsemiconductor laser based devices with even greater output optical powerand improved output efficiency.

SUMMARY OF THE INVENTION

The asymmetry of output reflectivity of the two facets of conventionalhigh-power lasers was found to give rise to a strong longitudinalspatial hole burning (LSHB) that suppresses optical gain for the laserlight at the output facet, increases the free-carrier optical absorptionat the back facet of the lasers, and limits the maximum achievableoutput power for long-cavity lasers. These photon and carrierlongitudinal inhomogeneities become more significant at higher currents,for longer cavities and with more asymmetric FF/BF coatings.

Accordingly, the present invention provides a bi-directionalsemiconductor laser having unfolded cavity with two AR-coatedreduced-reflectivity facets for reducing LSHB related limitations on themaximum achievable power and the optimal laser length.

More particularly, one aspect of the present invention provides along-cavity semiconductor laser device (SLD) for high-powerapplications, the SLD comprising a semiconductor laser chip whichcomprises first and second facets defining a laser cavity therebetween,and a laser waveguide extending between first and second facets andcomprising an active layer for generating laser light, wherein the lasercavity is at least 5 mm long, and wherein each of the first and secondfacets comprises anti-reflection coating that is configured forreflecting back into the laser cavity between 0.1% and 10% of the laserlight incident upon the respective facet, and for outputting at least90% of the laser light incident thereupon. The SLD may comprise beamcollimating optics for collimating the laser light emitted from thefirst and second facets into first and second collimated beams, and mayfurther comprise beam combining optics for combining the laser lightemitted from the first and second facets into a single output beam. Thebeam combining optics may include a polarization converter and apolarization beam combiner for combing the two collimated beams havingorthogonal polarization states into one combined beam that may besubstantially non-polarized.

A further aspect of the present invention relates to a light sourcecomprising a plurality of the bi-directional SLDs disposed in aplurality of vertically offset levels for producing a first compositebeam that is composed of vertically stacked first collimated beamsemitted from the first facets of the plurality of the SLDs, and a secondcomposite beam that is composed of vertically stacked second collimatedbeams emitted from the second facets of the plurality of the SLDs. Thefirst and second collimated beams may then be polarization-combined intoa single composite beam composed of vertically offsetpolarization-combined beams from the individual bi-directional SLDs.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings which represent preferred embodiments thereof, inwhich like elements are indicated with like reference numerals, andwherein:

FIG. 1 is a simplified plan view of a conventional semiconductor laserfor high-power applications having a single AR coated facet (1AR);

FIG. 2 is a graph illustrating simulated distributions of the photondensity (top panel) and the carrier density in the active layer (lowerpanel) along the laser cavity of the conventional semiconductor laser ofFIG. 1;

FIG. 3 is a graph illustrating simulated contributions of the forwardand backward propagating waves into the total photon densitydistribution for the conventional semiconductor laser of FIG. 1 nearlaser threshold;

FIG. 4 is a graph illustrating simulated contributions of the forwardand backward propagating waves into the total photon densitydistribution for the conventional semiconductor laser of FIG. 1accounting for longitudinally-nonuniform optical gain saturation at ahigher optical power;

FIG. 5 is a simplified plan view of a bi-directional semiconductor laseraccording to an embodiment of the present invention having two AR coatedfacet (2AR);

FIG. 6 is a cross-sectional view of an exemplary embodiment of thebi-directional semiconductor laser of FIG. 5;

FIG. 7 is a graph illustrating simulated contributions of the forwardand backward propagating waves into the total photon densitydistribution for the bidirectional semiconductor laser of FIG. 5;

FIG. 8 is a graph showing measured output power vs. injection currentcharacteristics of three semiconductor laser chips that differ only bytheir facet reflectivities: a conventional 1AR laser of FIG. 1 (401), abi-directional 2AR laser (402) with 4% output reflectivity of bothfacets, and a bi-directional 2AR laser (403) with 10% outputreflectivity of both facets;

FIG. 9 is a schematic block diagram of a bi-directional laser source foremitting two parallel collimated beams;

FIG. 10 is a schematic block diagram of a semiconductor laser devicewherein light from both facets of a bi-directional lasers is combinedinto a single collimated beam;

FIG. 11 is a schematic block diagram of a semiconductor laser devicewherein light from both facets of multiple bi-directional lasers iscombined into a single polarization-combined composite output beam;

FIG. 12 is a side view of the semiconductor laser device of FIG. 11;

FIG. 13 is a schematic cross-sectional view of a composite beam producedin the semiconductor laser device of FIGS. 11 and 12;

FIG. 14 is a schematic block diagram of a semiconductor laser devicewherein light from both facets of multiple bi-directional lasers iscombined into two collimated composite output beams.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular opticaldevices and circuits, circuit components, techniques, etc. in order toprovide a thorough understanding of the present invention. However, itwill be apparent to one skilled in the art that the present inventionmay be practiced in other embodiments that depart from these specificdetails. In other instances, detailed descriptions of well-knownmethods, devices, and circuits are omitted so as not to obscure thedescription of the present invention.

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives, modifications and equivalents, as willbe appreciated by those of skill in the art. The terms “horizontal” and“vertical” as used herein are with respect to the main plane of therespective device, such as with respect to the plane of the substrate ofa semiconductor laser, or with respect to a plane of the device supportbase. The terms “connect,” “couple,” “mount” and similar terms withtheir inflectional morphemes do not necessarily denote direct andimmediate connections, but also include connections through mediateelements or devices. The terms “first”, “second” and so forth are notintended to imply sequential ordering, but rather are intended todistinguish one element from another unless explicitly stated.

With reference to FIG. 1, a conventional high-power semiconductor laser(SL) 10 has a laser cavity that extends from the back facet (BF) 1 tothe front facet (FF) 3 of the laser chip. The width of an active area ofthe laser is defined by the width of a laser waveguide 15, and may be onthe order of a few microns (μm), 3-5 μm typically, for single lateralmode lasers, and may be tens or even hundreds microns for so calledbroad-area multi-mode devices. The back facet 1 of the SL 10 has an HRcoating 11, which is typically configured so that it reflects back intothe laser cavity about 99% or more of the laser radiation 31 that isgenerated within the laser cavity, i.e. its reflectivity R_(BF) is about99%. The front facet 3 has the AR coating 13 that is designed so as tosuppress the natural Fresnel reflectivity of the laser facet and letalmost all light 31 out of the laser cavity. Accordingly, SL 10 has a‘folded’ cavity, which is also referred to herein as the ‘1AR’ cavity,and is a substantially unidirectional device that emits light from onlythe front facet 3 of the laser chip. In practice, the front facetreflectivity R_(FF) of conventional high-power lasers is typically about1% or less and is optimized for maximum light output through FF 3, sothat about 99% of the laser light 31 incident upon FF 3 escapes thelaser cavity as the output light 18.

One disadvantage of the unidirectional cavity design is that itexacerbates the longitudinal spatial hole burning (LSHB) in the activeregion of the laser, which limits the maximum output power that may beachieved by increasing the cavity length of the laser. At very highoptical power in longer cavity devices the photons in the optical modedeplete the electron-hole pairs toward the output side of the laserdiode faster than the carriers can be injected, resulting in the LSHB.This causes strong decrease of carrier density and gain near the outputfacet FF 3, leading to reduction in photon density at FF 3 and hence alower output power. Furthermore, a carrier accumulation outside theactive layer(s) of the laser at and near BF 1 of the cavity causeshigher absorption losses at the BF 1.

The LSHB in a conventional unidirectional semiconductor laser withasymmetric facet coatings is illustrated in FIGS. 2-4, which showresults of computer simulations of the photon and carrier densityvariations for a simplified model of a semiconductor laser with a BFreflectivity R_(BF)=0.99, or 99%, and two values of the FF reflectivity:R_(FF)=0.1 (10%) and 0.01 (1%).

Referring first to FIG. 2, the top panel shows a simulated distributionof the photon density along the laser cavity of SL 10, from the BF 1(left) to the FF 3 (right), and the two values of the front facetreflectivity, 1% (solid line) and 10% (dashed line). The distribution ofthe total photon density along the cavity is highly asymmetric, with thephoton density at the FF 3 much greater than at the BF 1. The asymmetryin the photon density distribution increases with the lowering of the FFreflectivity.

Continuing to refer to FIG. 2, the lower panel shows the correspondingcarrier density distribution along the laser length for the two values,1% and 10%, of the FF reflectivity. The photon density in the cavity iscoupled to the carrier density through local optical gain, resulting inthe LSHB in the laser cavity. As can be clearly seen, the LSHB-relatednon-uniformity of the carrier density along the laser cavity ispronounced for the 1% FF reflectively, with the carrier concentrationrising toward the back facet, while the carrier density stays relativelyflat along the cavity with 10% FF reflectivity. The carrier densitybuild-up at the BF 1 in lasers with 1% FF reflectivity increases thefree carrier absorption of the laser light in the cavity and decreasesthe output power of the laser.

FIG. 3 shows, in addition to the simulated photon density distributionfor FF reflectively of 1% (solid curve), also separately the photondensity of the forward and backward propagating waves within the lasercavity (dashed curves), with the lower dashed curve showing thecontribution of the back propagating wave, i.e. from the FF to the BF.The photon density within the cavity is normalized so that the outputoptical power is 10 (in arbitrary units, a.u.). In the absence of gainsaturation, the intensity of the laser radiation increases exponentiallywhen traveling from one facet to another within the cavity due to activeregion gain. The light intensity of the forward propagating wave dropsupon reflection at the FF 3 by 99%, as most of the forward propagatingwave exits the laser resonator at the FF 3, and only 1% of it isreflected back into the laser cavity as the back propagating wave. Theback-propagating photons then travel the cavity length L twice, with thetotal photon travel distance 2 L in the ‘folded’ cavity of SL 10, beforeleaving the cavity through FF 3.

The simulation results shown in FIG. 3 correspond to a relativelylow-power regime, when the gain saturation by stimulated emission isrelatively weak. In contrast, FIG. 4 schematically illustrates theeffect of the gain saturation due to the LSHB on the output opticalpower, assuming that the local optical gain in the laser cavity dropswhen the photon density exceed 8 a.u. At a higher power, the opticalgain near the output facet 3 is suppressed due to the lack of carriers,so that the normalized output power drops from 10 a.u. to 9 a.u. in thissimple illustrative example.

Thus, the strong asymmetry in the facet reflectivity of conventionaluni-directional high-power lasers results in a decreased laserefficiency and reduced optical output due to a disbalance in the photonand carrier distributions within the laser cavity, i.e. the carrierdepletion and suppression of the optical gain at the FF, where thephoton density is high, and the additional optical loss due theincreased free carrier absorption at the highly-reflective BF. Thedetrimental effects on the laser efficiency of both these factors becomemore pronounced at higher injection levels and with increasing the laserlength L. The asymmetry of the photon and carrier density distributionscan be reduced by increasing the FF reflectivity from the currentlypreferred in the prior art 1% to, e.g. 10%, but such an increase in theFF reflectivity by itself would reduce the light output efficiency fromthe FF of the laser.

The present disclosure provides a solution to this problem byeliminating the HR coating of the BF 1 and replacing it with an ARcoating that reduces, instead of increasing, the back facet reflectivitybelow its ‘natural’ Fresnel value determined by the refractive indexdiscontinuity at the laser chip-air boundary. Advantageously, thereplacement of the HR coating of the BF with the AR coating enables toweaken the detrimental effects of the LSHB on the laser outputefficiency and maximum power by making the laser resonator moresymmetrical, which reduces the disbalance in carrier and photondistributions and their build-ups at the opposing facets.

With reference to FIG. 5, there is schematically illustrated SL 20 thatemploys an ‘unfolded’ bidirectional laser cavity according to anembodiment of the present invention. Similarly to SL 10, it includes alaser waveguide 15 that extends between two opposing facets 21, 23 ofthe laser chip 25, which are generally orthogonal to the waveguide 15and serve as end-mirrors that define the laser cavity of SL 20. Incontrast with SL 10, both facets 21 and 23 of the laser chip areAR-coated with AR coatings 41 and 43, and are referred to herein as thefirst and second facets, respectively. Accordingly, the laser cavityconfiguration of SL 20 will also be referred to herein as the 2ARcavity, and the SL 20 as the 2AR laser. The AR coatings 41, 43 aregenerally designed so that the reflectivity of each of the facets 21, 23is in the range of 0.1% to 10%, and preferably in the range of 0.3% to10% in embodiments absent of external cavity; techniques for designingand implementing such coatings for SL chips are well known in the artand will not be described herein. In one currently preferred embodimentthese reflectivities are approximately equal to each other to provide asymmetrical laser cavity, but may also differ to some extent in otherembodiments.

FIG. 6 illustrates a cross-sectional view of SL 20 in one exemplaryembodiment thereof, which corresponds to a cross-section A-A indicatedin FIG. 5. In the shown exemplary embodiment, SL 20 includes a shallowridge structure 33 that extends between facets 21, 23 and forms theoptical waveguide 15, with a stripe contact 16 disposed on top of theridge. The waveguide 15 incorporates a waveguiding layer structure 128disposed over a substrate 50, with an active layer 115 that may includeone or more quantum wells (QW) as known in the art. The ridge 33provides an optical continent in the lateral direction and guides thelaser light 31 generated in the active layer 115 to one of the facets21, 41. The term ‘lateral direction’ is conventionally used in the artof semiconductor lasers and refers to the direction that is parallel tothe main substrate plane of the laser chip 25 and normal to the lightpropagation direction within the waveguide 15. In other embodiments, theridge 33 may extend through one or more of the waveguiding layers 128formed in or upon the semiconductor substrate 55. In yet otherembodiments, the waveguide 15 may be of the buried-waveguide typewherein the ridge 33 is buried under additional layers of a lower-indexmaterial providing the lateral optical confinement of the spatial lasermode, as known in the art. In one embodiment, the waveguide 15 isdimensioned so that it is effectively single-mode in the lateraldirection. By way of example, in this embodiment the width of the ridgecan be in the range of 3 to 5 μm. In another embodiment, SL 20 may be abroad-area laser, with the effective width of the ridge 33 and/or thecurrent injection contact 16 ranging from tens to hundreds microns, forexample in the range of 80 to 200 microns, or in the order of 100 μmtypically. Although less preferred for some applications, an embodimentwherein ridge 33 is absent and the lateral optical confinement in thewaveguide 15 is defined entirely by the width of the current contact 16and the lateral profile of the resulting optical gain and carrierdistribution in the active region of the laser is also within the scopeof the present invention. Generally, the material composition, layerstructure and the lateral waveguiding structure of SL 20 may vary andmay be as known in the art for conventional high-power laser diodes.

Turning back to FIG. 5, in the ‘unfolded’ 2AR cavity design of SL 20, incontrast with the conventional ‘1AR’ cavity configuration of SL 10, mostof the photons generated within the cavity travel the cavity at mostonce before exiting it through one of the facets, with the correspondingphoton travel distance being ˜L instead of ˜2 L for SL 10. This reducesthe photon and carrier density buildup at the facets for 2AR lasers ofthe same length as 1AR devices, which results in a more uniform gainalong the cavity length, thereby limiting negative effects of LSHB onthe laser efficiency.

Referring to FIG. 7, dashed curves illustrate simulated intensities ofthe ‘forward’ and ‘backward’ propagating waves in the 2AR cavity of SL20, while the solid curve shows the resulting total photon density alongthe length of SL 20. These results were computed, using the samecomputer model as for FIGS. 2-4, for an exemplary case of 10%reflectivity of both laser facets, which is approximately equivalent tothe 99%/1% BF/FF reflectivities of SL 10 in terms of the total outputcoupling efficiency. Comparing to the photon density distribution ofFIG. 3, the unfolded cavity of SL 20 indeed results in a more uniformphoton density profile along the cavity, and therefor considerablyweaker LSHB.

Advantageously, the reduction in the LSHB makes it possible to increasethe length of the laser chip well beyond optimal length of 1AR SLs,which is typically about 5 mm, without the laser performance becominglimited by the adverse effects of the LSHB. As a further advantage, thefacets reflectivity may be decreased when laser length increases, whichincreases the out-coupling efficiency of the laser cavity, enabling anadditional improvement in the output laser efficiency and/or maximumoutput power.

Turning now to FIG. 8, the output power performance of twoexperimentally fabricated prototypes of the bidirectional (2AR) SL 20,which differ by their facets reflectivity, is shown in comparison tothat of the conventional uni-directional, or 1AR, SL 10 for injectioncurrents up to 40 A. All three lasers have the same length of 5.7 mm andsubstantially identical device structure, except for facet coatings.Curve 401 shows the FF output power P of the 1AR laser in dependence onthe injection current J, while curves 402 and 403 show the P(J)dependencies for the 2AR devices with equal facet reflectivity of bothfacets of 4% (402) and 10% (403). The output power curves 402 and 403show the total output power of the respective 2AR devices from bothfacets. The P(J) characteristic 401 of the standard 1AR SL exhibits anearlier “rollover” than those of the 2AR devices, and the 1AR devicefailed by COD (catastrophic optical damage) at the output power of about20 W. The equivalent 2AR device with R=10% coating on both facetsreached approximately 30 W of the output power without failure. For thesecond 2AR device, the AR coating of the facets was modified to reducethe facet reflectivity to 4%, which resulted in a yet higher totaloutput power of about 32 W due to the enhanced outcoupling of the lightgenerated within the cavity. Advantageously, the 2AR devices did notexhibit a complete P(J) rollover at the maximum power level. In theseexamples, the power improvement achieved in 2AR lasers, which weattribute at least in part to a reduction of LSHB, is about 50%.

Further investigations have shown that additional improvements inmaximum output power and output efficiency can be achieved by furtherincreasing the laser length L beyond 5 mm and by further decreasing thefacet reflectivity in longer-cavity devices. Generally, facetsreflectivity of useful 2AR lasers with cavity length of about 5 mm andabove should preferably be in the range from 0.3% to 5% for goodperformance, with optimal values depending on the cavity length and,possibly, other device specifics such as waveguide width and strengthand epitaxial layer structure. In one preferred embodiment, the facetreflectivity is in the range of 0.5% to 5%, for each facet, for deviceswith the laser cavity length L in the range from about 6 mm to 15 mm. Inone exemplary embodiment, an optimal facet reflectivity is about 1% to3% and the laser length is about 8 to 10 mm.

Referring now to FIG. 9, there is illustrated an exemplary laserarrangement 100 that is based on SL 20 and additionally includes opticsfor collimating the laser output beams 28 from the two facets andoptionally re-directing these beams to propagate in a same direction, asmay be useful in applications. This optics includes two sets of beamcollimating optics 210, 215, one for each facet, and two reflectors 40.

Light emitted from a high power SL is typically highly asymmetric due toa thin-slab geometry of their active regions and waveguides, resultingin long and thin emitting apertures at the laser facets. The light beamemitted by such lasers can be described in term of its ‘fast axis’ and‘slow axis’; it has a much higher brightness and divergence in thedirection of its “fast axis”, which is perpendicular to the active layerof the laser, than in the direction of its “slow axis”, which isparallel to the active layer. In FIG. 9, the active layer of SL 20, thelong axis of its output aperture, and the slow axis of the laser beams28 are all parallel to the plane of the figure, while the fast axis ofthe laser beam is normal to the plane of the figure.

Accordingly, in the shown embodiment the beam collimating optics foreach of the laser beams 28 includes a fast axis collimator (FAC) 210,which is preferably located in a close proximity to the respectivefacet, for collimating the laser beams 28 in the fast axis plane, and aslow axis collimator (SAC) 215 for collimating the laser beams 28 in theslow axis plane. The term “fast axis plane” as used herein refers to aplane defined by the fast axis of a beam and the beam's propagationdirection, and the term “slow axis plane” refers to a plane defined bythe slow axis of a beam and the beam's propagation direction. Withrespect to FIG. 9, the slow axis plane is parallel to the plane of thefigure, and the fast axis plane is orthogonal to the plane of thefigure.

The FACs 210 can be, for example, in the form of optical lenses that arecylindrical, or, more generally, toroidal in shape, such that while thedivergence of beam 28 in the fast axis plane is minimized or at leastsubstantially reduced so that the beams 28 are collimated in theirrespective fast axis planes after the FACs 210, the divergence of beams28 in the slow axis plane is nearly unaffected or affected to a lesserdegree. SACs 215 may be disposed further away from SL 20, either beforeor after the reflectors 40, and may each be in the form of a cylindricalor toroidal lens as known in the art. The function of SACs 215 is tocollimate the beams 28 such that after SACs 215 the beams 28 arecollimated in both the slow axis plane and the fast axis plane. Thedistance between each SAC 215 and the corresponding facet may beselected so as to provide the beam 28 with a desired aspect ratio. Inother embodiments SAC 215 and FAC 210 disposed in the optical path ofthe same beam 28 may be embodied using a single lens element of asuitable shape that is selected so as to collimate the laser beam 28 inboth the fast and slow axis planes.

The reflectors 40 are beam turning elements that are disposed in theoptical path of the respective laser beams 28 so as to direct said beamsin a same general direction in parallel. Each of the reflectors 40 maybe embodied as a flat mirror or other beam-deflecting device, such asbut not limited to an optical prism or a diffraction grating. It will beappreciated that there are more than one way to position reflectors 40to direct beams 28 in the same general direction, and FIG. 9 shows justone exemplary positioning of these reflectors. For example, bothreflectors can be sequentially positioned in the optical path of thesame beam 28, turning it by 180°.

Referring now to FIG. 10, there is illustrated an exemplary embodimentof a laser source 200 that includes the laser arrangement 100 andadditionally includes optics for combining the collimated laser outputbeams 28 from the two facets into a single output beam 55. Thisadditional optics includes a third beam reflector 79, a polarizationconverter 77, and a polarization beam combiner (PBC) 86. The PBC 86 isdisposed in the optical path of one of the collimated beams 28, and thereflector 79 is a beam turning elements that is disposed in the opticalpath of the other of the laser beams 28 so as to direct said beamtowards the polarization beam combiner 86 for combining it with thelaser beam 28 from the other laser facet into a single output beam 55.The reflector 79 may be embodied as a flat mirror or otherbeam-deflecting device, such as but not limited to an optical prism or adiffraction grating. The polarization converter 77 may be disposed inthe optical path of any of the beams 28 to convert its polarizationstate to an orthogonal one.

Turning now to FIGS. 11 and 12, there is illustrated, in plane and sideviews respectively, another embodiment of the semiconductor laser device(SLD) of the present disclosure in the form of an apparatus 300, whichcombines light collected from both facets of a plurality of Nbidirectional SLs 20 into a composite light beam 155 suitable forcoupling into an optical fiber. FIG. 11 illustrates the plane view ofthe SLD 300, while FIG. 12 shows the side view thereof taken in thedirection of arrow ‘A’ shown in FIG. 11.

In this embodiment, the apparatus 300 may be viewed as N bidirectionallaser arrangements 100 of FIG. 9 that are disposed in a plurality ofvertically offset levels so as to produce a first composite beam 81 athat is composed of vertically stacked first collimated beams 28 aemitted from the first facets of the plurality of the SLs 20, and asecond composite beam 81 b that is composed of vertically stacked secondcollimated beams 28 b emitted from the second facets of the plurality ofthe SLs 20. SDL 300 is a modification of the beam combining light sourcethat is disclosed in U.S. Pat. Nos. 8,427,749 and 8,437,086, which areincorporated herein by reference, except that it replaces pairs ofco-planar uni-directional lasers disposed in a same level with a singlebi-directional SL 20.

In the shown exemplary embodiment N=3, so that the apparatus 300 iscomprised of three instances of the bidirectional SL 20, namely SL 20 a,SL 20 b, and SL 20 c, which are mounted upon different “steps” 13 a-13 cof a stepped support base 211. Each SL 20 is provided with beamcollimating and turning optics 210, 215, 40 for each facet thereof, asdescribed hereinabove with reference to FIG. 9, with steps 13 a-13 chaving a non-zero height h that preferably exceeds the verticaldimension of the collimated beams 28 a,b from individual SLs 20. In oneembodiment, each of the steps has a single one of the SLs 20 mountedthereon with the planes of slow axes thereof being parallel to themounting surface.

The reflectors 40 align beams 28 a,b emitted from similarly-orientedfacets of SLs 20 in the plane of the support base 211 to form twocomposite laser beams 81 a and 81 b, as illustrated in FIGS. 11 and 12with dashed lines. Each of the first and second composite beams 81 a, 81b is composed of the first (28 a) or second (28 b) collimated beams,respectively, that are stacked in a direction of the fast axes thereof,as illustrated in FIG. 13. Thus, each of the composite laser beams 81a,b is composed of N vertically stacked collimated SL beams 28 a or 28 bemitted from SL facets facing the same direction, with N=3 in FIGS.11-13. The composite beams 81 a and 81 b are then combined using the PBC86, the turning mirror 79, and the polarization converter 85 to form apolarization-combined output composite beam 155, which is composed of Nvertically stacked collimated beams from individual SLs 20, each ofwhich in turn composed of overlapping and polarization-combined beams 28a or 28 b emitted from both facets of the same SL 20. Thepolarization-combined output composite beam 155 may then be coupled intoan optical fiber 99 using a coupling optics 88, such as a suitablecoupling lens, for example convex-convex or convex-plane, or ananamorphic telescope as known in the art.

Referring now to FIG. 14, in another embodiment the polarizationconverter 85, PBC 86 and the turning mirror 79 of SLD 300 may beomitted, and instead the two composite beams 81 may be coupled into twooutput fiber ports 99 a and 99 b using coupling optics 88 a and 88 b, soas to provide a high-power optical source with two output beams, eachcomposed of a plurality of parallel vertically offset and horizontallyaligned collimated optical beams.

Advantageously, SLD 300 and SLD 400 require significantly fewerindividual lasers to provide the same optical power in the output fiber99 as the multi-laser optical sources disclosed in U.S. Pat. Nos.8,427,749 and 8,437,086, thereby simplifying optical alignment andreducing the cost of the device per unit of output power.

The above-described exemplary embodiments are intended to beillustrative in all respects, rather than restrictive, of the presentinvention. Thus the present invention is capable of many variations indetailed implementation that can be derived from the descriptioncontained herein by a person skilled in the art. All such variations andmodifications are considered to be within the scope and spirit of thepresent invention as defined by the following claims.

1-14. (canceled)
 15. A semiconductor laser device comprising: asemiconductor laser chip comprising: a first facet and a second facetdefining a laser cavity therebetween; and a laser waveguide, extendingbetween the first facet and the second facet, comprising an active layerfor generating laser light, wherein the first facet and the second faceteach include an anti-reflection coating that is configured forreflecting back into the laser cavity a first portion of the laser lightand for outputting a second portion of the laser light, and a buildup ofa carrier density near the first facet and the second facet is reduced,thereby providing for greater gain uniformity along the laser cavity.16. The semiconductor laser device of claim 15, wherein at least 90% ofthe laser light travels the laser cavity one time before exiting throughthe first facet or the second facet.
 17. The semiconductor laser deviceof claim 15, further comprising: a ridge that extends between the firstfacet and the second facet.
 18. The semiconductor laser device of claim15, wherein the laser waveguide has a width in a range of: 3 to 5microns, or 80 to 200 microns.
 19. The semiconductor laser device ofclaim 15, wherein the laser cavity is at least 5 mm long.
 20. Thesemiconductor laser device of claim 15, further comprising: first beamcollimating optics for collimating laser light, emitted from the firstfacet, into a first collimated beam; and second beam collimating opticsfor collimating laser light, emitted from the second facet, into asecond collimated beam.
 21. The semiconductor laser device of claim 20,further comprising: a first reflector for reflecting the firstcollimated beam; and a second reflector for reflecting the secondcollimated beam.
 22. The semiconductor laser device of claim 21, furthercomprising: a beam combiner for combining the reflected, firstcollimated beam and the reflected, second collimated beam into a singleoutput beam.
 23. An apparatus comprising: a plurality of semiconductorlaser devices, wherein each semiconductor laser device, of the pluralityof semiconductor laser devices, includes a semiconductor laser chipincluding: a first facet and a second facet defining a laser cavitytherebetween; and a laser waveguide, extending between the first facetand the second facet, comprising an active layer for generating laserlight, wherein the first facet and the second facet each include ananti-reflection coating that is configured for reflecting back into thelaser cavity a first portion of the laser light and for outputting asecond portion of the laser light, and a buildup of a carrier densitynear the first facet and the second facet is reduced, thereby providingfor greater gain uniformity along the laser cavity.
 24. The apparatus ofclaim 23, wherein at least 90% of the laser light travels the lasercavity one time before exiting through the first facet or the secondfacet.
 25. The apparatus of claim 23, wherein the laser cavity is atleast 5 mm long.
 26. The apparatus of claim 23, further comprising:first beam collimating optics for collimating laser light, emitted fromthe first facet, into a first collimated beam; and second beamcollimating optics for collimating laser light, emitted from the secondfacet, into a second collimated beam.
 27. The semiconductor laser deviceof claim 26, further comprising: a first reflector for reflecting thefirst collimated beam; and a second reflector for reflecting the secondcollimated beam.
 28. The semiconductor laser device of claim 27, furthercomprising: a beam combiner for combining the reflected, firstcollimated beam and the reflected, second collimated beam into a singleoutput beam.
 29. A method comprising: generating, using an active layer,laser light, wherein the active layer is located within a laserwaveguide of a semiconductor laser chip that includes a first facet anda second facet defining a laser cavity therebetween, the laser waveguideextends between the first facet and the second facet, and the firstfacet and the second facet each include an anti-reflection coating;reflecting, using the first facet and the second facet, a first portionof the laser light; and outputting, using the first facet and the secondfacet, a second portion of the laser light, wherein a buildup of acarrier density near the first facet and the second facet is reduced,thereby providing for greater gain uniformity along the laser cavity.30. The method of claim 29, wherein at least 90% of the laser lighttravels the laser cavity one time before exiting through the first facetor the second facet.
 31. The method of claim 29, wherein the laserwaveguide has a width in a range of: 3 to 5 microns, or 80 to 200microns.
 32. The method of claim 29, wherein the laser cavity is atleast 5 mm long.
 33. The method of claim 29, further comprising:collimating, using first beam collimating optics, laser light, emittedfrom the first facet, into a first collimated beam; and collimating,using second beam collimating optics, laser light, emitted from thesecond facet, into a second collimated beam.
 34. The method of claim 33,further comprising: combining, using a beam combiner, the firstcollimated beam and the second collimated beam into a single outputbeam.